Such mass flow meters have long been known. The measuring principle of thermal mass flow meters is based on the cooling of a heating element mounted on a holder when immersed into a flowing fluid. The flow which flows over the surface of the heating element absorbs heat from the latter and thus cools the heating element. The construction and behavior are illustrated in principle in FIG. 3. In this case, the quantity of heat absorbed by the flow depends on the temperature difference between the surface and the fluid, and on the flow itself. It can be described by a function{dot over (q)}=α(TO−TF),where    {dot over (q)} is the quantity of heat dissipated,    (TO−TF) is the temperature difference, and    α is a constant of proportionality.
The constant of proportionality α is in this case directly dependent on the flow and is a function of the mass flow density over the heating element α=ƒ(ρν)˜√{square root over (ρν)}. Now, if the temperature difference between the surface and the fluid, and also the heating power required to generate this temperature difference, are known, the mass flow over the heating element can thus be determined from this.
Therefore, for practical application of such a thermal mass flow measurement, two temperature sensors, one of which is heated and used for the flow measurement, are now put into the flow as illustrated in FIG. 4. The second temperature sensor serves to measure the fluid temperature TF.
In general, the measurement is in this case carried out only statically with a constant heating power or a constant temperature difference between the heater and the flow. However, a pulsed mode of operation, which is evaluated with slightly more effort, could also be carried out in this case.
However, for all these measurements here it is important that a very accurate measurement of the heating power and the temperature difference is carried out. The quantity of heat given off to the flow can not be measured directly in this case but is rather determined by measuring the electrical heating power used. However, due to the construction, the electrical heating power introduced is not completely given off to the flow directly from the sensor head but a part of the heat flows into the holder of the sensor head and from there it is given off to the surroundings or to the flow at a greater distance from the measuring element. Since this heat flux is included in the measurement of the mass flow, it directly influences the measured result and presents a great source of error when using a thermal mass flow meter. It is partially taken into consideration during the calibration of the mass flow meter. However, since it is very variable, depending in particular on the flow and temperature conditions in the flow, it can be considered only to a limited extent during calibration and thus still presents a great source of error. It is thus attempted to keep this heat-loss flux as low as possible during the development of a thermal mass flow meter in order to achieve a flow measurement that is as accurate as possible.
In order to reduce this influence, it is generally attempted to set the ratio of the direct heat flux into the flow and the losses into the holder to be as great as possible during the development of a thermal mass flow meter. That is to say, a very good thermal contact between the heater and the flow is created and, at the same time, the heat outflow into the holder is reduced by appropriate insulation. A possible embodiment is presented in U.S. Pat. No. 5,880,365. In general, the insulation in this case comprises the entire holder of the sensor head in order to create the best insulation possible.
The insulation achievable is however limited by the mechanical requirements of the holder. By way of example, a very good insulator is a tube filled with air with the thinnest wall thickness possible, since gasses have a much lower thermal conductivity than liquids or solids. However, since the holder in general is subjected to relatively high pressures and also has to hold the sensor head fixedly in its position, the reduction of the wall thickness is limited and often appropriate stabilizers are also installed into the holder, such as, for example, short solid cylinders which are arranged at regular intervals, as a result of which the insulation is decreased. Thus a residual heat-loss flux remains.
Other concepts thus go down the route of measuring the temperature at one or more points in the holder. Subsequently, the heat-loss flux is calculated with the aid of theoretical models of the holder and the sensor head, and this is considered during the determination of the mass flow. However, this concept requires one or more additional temperature measurements for this purpose, which creates increased effort and corresponding costs both in the sensor head and in the attached electronics.
Another disadvantage in this case is that the thermal conductivity inside the holder can be described well theoretically, but a large proportion of the heat is given off to the flow via the surface of the holder. This quantity of heat that is given off depends directly on the local temperatures and the flow velocities of the fluid around the holder, which in general are known only very imprecisely. In particular in the case of very large temperature differences between the housing and the flow, this is difficult to determine, so that the achievable accuracies are also limited as a result.